1. Field of the Invention
The present invention relates to a compact lens system capable of forming images with minimal aberrations, even when the medium which transmits light to the front surface of the lens system is changed, for example, from air to water.
2. Description of the Prior Art
Photography, in an environment other than air, has been suggested in the prior art for a considerable period of time. An example of such work is disclosed in an article published in Photographic Industries on July 1, 1973, entitled "Optical Problems Under Water" by Shimizu et al.
Attempts have been made in the medical field of endoscopes to accommodate them to both a gas and liquid environment as set forth in the Laid-Open Japanese Patent Application No. 50-21749 (1975).
As is well known, the aberrations of a lens system generally change if the medium, which transmits light to the lens system, is changed since the refractive index and Abbe number at the object side of the front surface of the lens system changes. For example, the aberration correction of a lens system which is achieved for exclusive use in air would be greatly disturbed if the lens system is used under water, or the aberration correction of a lens system which is achieved for exclusive use under water would be greatly disturbed if the lens system is used in air.
Therefore, suggestions in the prior art to design a lens system for dual use in air and water have been to take a mean course between aberration corrections for exclusive use in air and for exclusive use under water. Thus, the degree of disturbance of aberrations from a fine corrected condition would be decreased to a half in comparison with the case of aberration correction with respect to exclusive use in air if the lens system is used under water, and vice versa. Such lens systems show an almost identical optical performance both in air and water. It is apparent, however, that the optical performance of such a lens system cannot be superior to an exclusive lens system which is designed for use only in air or under water.
Additionally, the above described prior art would be only successful when the field angle of the lens system is narrow and the lens system is of limited speed, since disturbance of aberration correction is tolerable under such conditions.
The following patents are cited of general interest, British Pat. No. 1,238,668, U.S. Pat. No. 3,904,275, the Japanese publicly available published patent application disclosure No. 48-2823, and the German Offenlegungsschrift No. 2224429.